Enable Snappy compression support in LevelDb in cmake builds

[dconeybe]

Enable Snappy compression support in LevelDb databases when compiled via cmake. This does not affect builds from xcode.

Snappy support is mainly required for firebase-unity-sdk. All builds of the Unity SDK to date had inadvertently included Snappy support by virtue of being compiled in Google's internal build environment. Builds of the Unity SDK from GitHub, however, did not include Snappy support. Therefore, if a customer upgraded from a version of the Unity SDK that included Snappy support to a version that does not, they would experience a crash like this:

ERROR: FIRESTORE INTERNAL ASSERTION FAILED: /Users/[redacted]/Documents/GitHub/firebase-unity-sdk/macos_unity/bin/external/src/firestore/Firestore/core/src/local/leveldb_transaction.cc(76) void firebase::firestore::local::LevelDbTransaction::Iterator::Seek(const std::string &): leveldb iterator reported an error: Corruption: corrupted compressed block contents (expected db_iter_->status().ok())

The crash occurs because LevelDb tries to read a database that is compressed with Snappy, but Snappy compression support is not compiled in.

Note that only desktop platforms are affected by this crash: Windows, macOS, and Linux. Namely, the mobile platforms are not affected: Android, iOS, and watchOS. Android is not affected because it uses sqlite, not leveldb. iOS and watchOS are not affected because they get their LevelDb dependency from CocoaPods/Swift Package Manager, which never included Snappy support.

This PR was patched into the C++ SDK via firebase/firebase-cpp-sdk#885. This patching will be undone via firebase/firebase-cpp-sdk#932.

Googlers see b/227782613 for more details.

#no-changelog

[google-oss-bot]

Coverage Report ¹

Affected Products

• FirebaseFirestore-iOS-FirebaseFirestore.framework

  Overall coverage changed from 88.12% (8d65537) to 88.09% (a3ec861) by -0.03%.

  Filename	Base (8d65537)	Merge (a3ec861)	Diff
  exception.cc	84.21%	23.68%	-60.53%
  exception_apple.mm	65.52%	96.55%	+31.03%
  leveldb_key.cc	98.09%	98.79%	+0.70%

Test Logs

• Base (8d65537): https://github.com/firebase/firebase-ios-sdk/actions/runs/2322142956
• Merge (a3ec861): https://github.com/firebase/firebase-ios-sdk/actions/runs/2334152110

1. https://storage.googleapis.com/firebase-sdk-metric-reports/ucdlKC36cc.html

[dconeybe]

Some of the checks are failing because cmake's FindPythonInterp is finding a Python2 interpreter instead of Python3. I'm going to use the newer FindPython3 module, if available, to hopefully fix this issue.

[google-oss-bot]

Size Report ¹

Affected Products

• FirebaseFirestore

  Type	Base (e376c29)	Merge (6ba4ff9)	Diff
  CocoaPods	?	6.87 MB	? (?)

Test Logs

• Merge (6ba4ff9): https://github.com/firebase/firebase-ios-sdk/actions/runs/2296069676

1. https://storage.googleapis.com/firebase-sdk-metric-reports/0iLdtDjbIQ.html

[jonsimantov]

Reviewing - in the meantime, WDYT about getting the C++ SDK PR drafted (temporarily referencing this branch as its Firestore iOS SDK external cmake source) to make sure the build still works?

[dconeybe]

Reviewing - in the meantime, WDYT about getting the C++ SDK PR drafted (temporarily referencing this branch as its Firestore iOS SDK external cmake source) to make sure the build still works?

Done: commit 346a4a4109 from #932

[cherylEnkidu]

Thank you for your changes and the explanations!